Electrochemical method for the preparation of metal borohydrides



Oct. 7, 1958 G. F. HUFF ETAL 2,855,353

ELECTROCHEMICAL METHOD FOR THE PREPARATION OF METAL BOROHYDRIDES Filed April 12. 1954 INVENTORS 660.46! E. A l/FF 4486477 0. ueamr BY ear M 4044/ M Macaw/4m ELECTROCHEMICAL METHOD FOR PREPA- RATION OF METAL BOROHYDRIDES George F. Hull, Fox Chapel, Albert D. McElroy, Evans City, and Roy M. Adams, Darlington, Pa., assignors to Callery Chemical Company, Pittsburgh, Pa., a corporation of Pennsylvania Application April 12, 1954, Serial No. 422,647

7 Claims. (Cl. 204-59) This invention relates to a new and improved method for the preparation of metal borohydrides and more particularly to a new and improved method for preparing a desired metal borohydride in substantially quantitative yields from another metal borohydride by electrochemical displacement of one metal by another metal.

The borohydrides in general have received considerable attention in recent years because of their importance as selective reducing agents in chemical synthesis and as a readily available source of hydrogen. Sodium and potassium borohydrides are the only borohydrides readily available commercially. Magnesium borohydride is of special interest because of its high hydrogen content per pound. There are several methods for producing metal borohydrides one of which is the reaction of a metal alkyl with diborane. Another method consists of the reaction of a metal hydride with methyl borate while still another method utilizes a meta-thetic reaction between a metal borohydride and a metal halide. These methods have disadvantages such as the use of starting materials which are either expensive or not readily available, relatively low yields or complete inoperativeness of some of these methods in the preparation of certain borohydrides.

It is one of the objects of this invention to provide a new and improved method for the preparation of metal borohydrides in substantially quantitative yields from readily available borohydrides such as sodium, potassium or lithium borohydride.

Another object of this invention is to provide an improved method for preparing metal borohydrides from sodium borohydride by electrochemical displacement of one metal by another metal.

A further object is to provide an improved method for preparing metal borohydrides in which sodium borohydride is dissolved in an ionizing non-aqueous solvent and electrolyzed using a mercury cathode and a metal anode of the metal borohydride desired.

Other objects of this invention will become apparent from time to time throughout the specification and claims as hereinafter related.

This new and improved process will be described more fully in the specification and the novelty thereof will be particularly pointed out and distinctly claimed.

In the accompanying drawing there is shown a diagrammatic view of the apparatus in which this process is carried out.

In this invention a solution of an ionic metal borohydride MBH, in a non-aqueous solvent is electrolyzed using a mercury cathode and a sacrificial anode comprised of the metal (M of the borohydride desired. The metal M must form an amalgam les's readily than does the metal M. The metal M is deposited at the cathode as an amalgam which can be recovered by any suitable means. At the same time metal M ions will go into solution from the anode. If the desired borohydride M BH is substantially insoluble in the solvent, the M and BH, ions will react and the desired borohydride ice M BH, will precipitate at the anode. The term ionic metal borohydride as used herein is intended to refer to metal borohydrides which dissociate ionically in solution and which in solution will conduct an electric current.

When a solution of sodium borohydride in an appropriate solvent is electrolyzed using, for example, a magnesium anode and a mercury cathode the following electrode reactions occur:

Anode: Mg=Mg+++2e Cathode: Na++e +Hg=Na(Hg) The overall reaction is:

The net result of the electrolysis is the replacement of sodium ions with magnesium ions which are immediately precipitated as magnesium borohydride. The magnesium borohydride may be recovered by separating the same from the solvent. It should be noted that when this invention is carried out in a solvent in which the resultant borohydride is soluble the reaction will go to completion only if the metal ion of the resultant borohydride is higher in the replacement series with respect to its amalgam than is the sodium ion with respect to its amalgam.

In one experiment, magnesium borohydride was prepared by dissolving 0.395 mol of sodium borohydride in 300 ml. of liquid ammonia in an H-type cell. Five-tenths faraday of electricity was passed between the mercury cathode and magnesium anode which were placed in the two compartments of the cell. The solution was refluxed at 33 C. during the electrolysis. The initial current was two amperes with 30 applied volts but at the end of the experiment the current dropped to 50 milliamperes. The decrease in current was due to the slight solubility of the product, magnesium borohydride, in liquid ammonia. The anode and cathode current densities at the start of the electrolysis were 25 ma. per cm. and 100.ma. per cm. respectively. The cell was then evacuated at room temperature and a finely divided white solid was recovered therefrom and weighed and analyzed. The product was magnesium borohydride hexammoniate [Mg(BH -6NH and the yield was 98 percent. Four moles of ammonia were readily removed from the product by evacuation of the same at 100 C. and more complete removal of the ammonia may be effected at higher temperatures.

In another experiment, calcium borohydride was prepared by electrolyzing a solution of 10 g. of sodium borohydride in methylamine at the reflux temperature of the solution (-6 C.) using a calcium anode and a mercury cathode. A quantity of electricity approximately 20 percent in excess of that required for complete conversion of the sodium borohydride was passed through the cell. A solid product precipitated from the solution and was washed with methylamine, recrystallized from liquid ammonia and subjected to evacuation at room temperature to remove the solvent therefrom. Analysis indicated the product was calcium borohydride diammoniate 3 sodium borohydride is added continuously to a liquid ammonia solution which is continuously electrolyzed. The insoluble product is recovered at intervals by any suitable means such as filtration, and washed with liquid ammonia to separate the product from the sodium borohydride. The desired borohydride, however, must be stable and in the case of less electropositive metals it must be insoluble in the electrolytic solvent. Solvents which may be employed are ammonia, lower organic amines, such as methylarnine, ethylamine, dimethylamine, and pyridine and higher ethers such as the polyethylene glycol dimethyl ethers [CH O(C H O),,CH Metals which may be used as anodes include any desired metal the amalgam of which is more electropositive than sodium amalgam, or whose borohydride is insoluble in the solvent used for the electrolysis.

It is possible to use borohydrides of metals other than sodium as the solute in this process. The necessary requisites are that the dissolved borohydride must dissociate ionically in the solvent, and the resultant borohydride must be insoluble in the solvent or the amalgam of the metal anode must be more electropositive than the amalgam of the metal deposited by electrolysis of the dissolved borohydride. It is apparent therefore that, subject to the aforementioned requisites, this process may thus be employed as a method of preparation of any desired metal borohydride from another metal borohydride.

While we have described a few embodiments of our invention it is to be understood that within the scope of the non-aqueous solution of an ionic alkali metal borohydl'ide using a mercury cathode and an anode selected from the group consisting of magnesium and calcium, in a non-aqueous solvent in which the resultant alkaline earth metal borohydride and said anode are substantially insoluble, and recovering the alkali metal borohydride precipitated at said anode.

2. A method according to claim 1 in which the nonaqueous solvent is a polyethylene glycol dialkyl ether.

3. A method according to claim 1 in which the nonaqueous solvent is of the group consisting of liquid ammonia and lower organic amines.

4. A-method according to claim 1 in which the ionic metal borohydride is selected from the group consisting of sodium, potassium, and lithium borohydrides.

5. A method of preparing magnesium borohydride comprising electrolyzing a solution of sodium borohydride in liquid ammonia using a mercury cathode and a magnesium anode and recovering the borohydride precipitated at the anode.

6. A method of preparing calcium borohydride comprising electrolyzing a solution of sodium borohydride in methylamine using a mercury cathode and a calcium anode and recovering the borohydride precipitated at the anode.

7. A method of preparing an alkaline earth metal borohydride selected from the group consisting of calcium and magnesium borohydrides comprising electrolyzing a non-aqueous solution of sodium borohydride using a mercury cathode and a sacrificial metal anode selected from the group consisting of magnesium and calcium to thereby produce said alkaline earth metal borohydride and a sodium amalgam, and recovering the borohydride thus formed, said anode being insoluble in said solvent.

No references cited. 

1. A METHOD OF PREPARING AN ALKALINE EARTH METAL BOROHYDRIDE SELECTED FROM THE GROUP CONSISTING OF CALCIUM AND MAGNESIUM BOROHYDRIDES COMPRISING ELECTROLYZING A NON-AQUEOUS SOLUTION OF AN IONIC ALKALI METAL BOROHYDRIDE USING A MERCURY CATHODE AND AN ANODE SELECTEDD FROM THE GROUP CONSISTING OF MAGNESIUM AND CALCIUM, IN A NON-AQUEOUS SOLVENT IN WHICH THE RESULTANT ALKALINEE EARTH METAL BOROHYDRIDE AND SAID ANODE ARE SUBSTANTIALLY INSOLUBLE, AND RECOVERING THE ALKALI METAL BOROHYDRIDE PPRECIPITATED AT SAID ANODE. 